Patients with mononucleosis can become contagious the moment they contract the Epstein-Barr virus, reports TeensHealth. Following contraction, the virus remains dormant in patients' bodies for the rest of their lives and can infect others if it resurfaces. However, people who have previously had mono are not always contagious.
The Epstein-Barr virus has an incubation period of approximately four to seven weeks, during which time patients may be unaware that they are contagious, explains TeensHealth. As a result, they may not take the necessary precautions to avoid infecting others. Some people are carriers of the virus but never experience symptoms to alert them of its presence.